Vitaly Churkin, the Russian ambassador to the United Nations, died unexpectedly in New York on Monday, the Russian Foreign Ministry confirmed. He was 64. The cause of death was initially unclear. Churkin had served as the country’s representative at the U.N. since 2006. In the 1990s, he served as Russia’s ambassador to Belgium under President Boris Yeltsin. Churkin often sparred with the United States at Security Council meetings, particularly over the issue of Russia’s involvement in the Syrian civil war. Samantha Power, who most recently served as the U.S. ambassador to the U.N. under President Barack Obama, posted a condolences message on Twitter. “Devastated by passing of Russian UN Amb Vitaly Churkin. Diplomatic maestro & deeply caring man who did all he [could] to bridge [U.S.-Russian] differences,” she wrote.